The present invention relates to furnaces and particularly to apparatus for automatically cleaning ports introducing combustion air into the firebox of a chemical recovery furnace.
Wood pulp for papermaking is usually manufactured according to the sulfate process wherein wood chips are treated with a cooking liquor including sodium sulfide and sodium hydroxide. The wood chips and the cooking liquor, called "white liquor," are cooked in a digester under predetermined heat and temperature conditions. After cooking, the used liquor, termed "black liquor," containing spent cooking chemicals and soluble residue from the cook, is washed out of the pulp and treated in a recovery unit where the cooking chemicals are reclaimed. Without reclamation and reuse of the cooking chemicals, the cost of the papermaking process would be prohibitive.
In the recovery process, the black liquor is first concentrated by evaporation to a water solution containing about 65 percent solids, which solution is then sprayed into the firebox of a black liquor recovery boiler, a type of chemical reduction furnace. The chemical reduction furnace is a reactor wherein the processes of evaporation, gasification, pyrolysis, oxidation and reduction all occur interdependently during recovery of the cooking chemicals. The organic materials in the black liquor, lignin and other wood extracts, maintain combustion in the firebox, and the heat produced dries and melts the spent cooking chemicals as they fall to the floor of the firebox, where they build a mound of material called a char bed. The char bed is further heated to further liquify the chemicals into a molten smelt that flows out of the furnace through a smelt spout to a collection tank. Concurrently, combustion heat is employed to generate steam in a water wall of the boiler for use as process steam and for generating electricity.
The combustion process requires the introduction of large volumes of air into the firebox, air comprising about 80 percent of the material entering the furnace. The air is forced into the firebox from wind boxes or ducts disposed at several levels in surrounding relationship to the firebox, through a plurality of air ports in the walls of furnace, viz.: primary, secondary and tertiary air ports. The primary air ports, through which about 40 to 50 percent of the air enters the furnace, are disposed on the side walls of the firebox near the bottom of the surface close to the char bed. The secondary air ports, through which about 35 percent of the air enters the furnace, are disposed around the walls of the firebox, higher than the primary air ports, and below the entry conduits through which the black liquor is sprayed into the firebox. While the primary air ports provide a relatively large volume of air with considerable turbulence for maintaining a fireball in the char bed, the secondary air ports provide a finer control and distribution of air above the char bed and distribute the air evenly in the black liquor spray to support the combustion thereof.
The black liquor sprayed into the firebox, having a consistency like warm 60 weight oil, swirls, burns and falls toward the bottom of the firebox as combustion products comprising char material and smelt. The smelt and char material contact the outer walls of the firebox and, cooled by the inflowing air, form excrescent deposits around edges of the air ports, particularly along the top edges of the secondary air ports where the excrescent material builds up and outward under influence of air rushing through the secondary air port. Such buildup of char material can block air flow through a port by as much as ten percent. In accordance with customary practice, the char buildup is periodically removed by a worker inserting a rod into the air ports successively around the boiler. With manual rodding of the air ports, gradual buildup of char material intermittently around the furnace causes changes in the volume of combustion air, as well as changes in air distribution, velocity and pressure. Therefore, furnace operation tends to be inefficient and unpredictable with an attendant decrease in the amount of chemicals that can be recovered, a decrease in the amount of steam produced per unit of fuel, and increased emission of noxious gases such as carbon monoxide and sulfur dioxide.
Apparatus for cleaning openings in a recovery furnace are known--see my U.S. Pat. No. 4,423,533, entiled FURNACE AIR PORT CLEANER; however, such known apparatus is not always suitable for cleaning secondary air ports. Conventional punching devices, rods or cleaners inserted into the secondary air ports cannot dislodge the buildup of material above the top edge of the air port.
Monitoring devices such as pyrometers for sensing temperature inside the firebox and television cameras for viewing conditions in the furnace are often installed in the secondary air ports of recovery furnaces. Heretofore it has been necessary to remove such devices in order to clean deposits of excrescent material which build up around the secondary air ports interiorly of the firebox.
It is accordingly an object of the present invention to provide improved apparatus for cleaning air ports of a chemical recovery furnace.
It is another object of the present invention to provide improved air port cleaning apparatus installable in an air port of a chemical recovery furnace without interference to a sensing device associated with the air port in which the cleaning apparatus is installed.
Another object of the present invention is to provide improved air port cleaning apparatus for increasing the operational stability of a black liquor recovery boiler.
It is a further object of the present invention to provide improved air port cleaning apparatus for enhancing the efficiency of chemical recovery, increasing steam production, and reducing sulfur dioxide and carbon monoxide emissions from a black liquor recovery boiler.